Rotational molding, also referred to as rotomolding, is widely used to produce hollow articles such as toys, sporting equipment, containers, water tanks, etc. For the process, a thermoplastic resin is placed in a mold which is then closed, heated and rotated on two axes, i.e., biaxially, to allow the resin to melt and uniformly coat the interior of the mold. The mold is then cooled and the molded article is removed.
In many instances, it is highly desirable to have a foam layer or core in the interior of the molded article to provide insulation, impart structural integrity or stiffness to the article, reduce weight, or the like. This is accomplished by including a foaming or blowing agent with the resin which decomposes at the molding temperature to release a gas, such as CO.sub.2 or N.sub.2.
The use of foaming agents presents a problem where formed articles having a smooth exterior surface are desired and various techniques have been employed to produce foamed rotomolded goods having a smooth skin layer. In one approach, referred to as the "two-step" method, a non-foamable resin is first introduced into the mold and molded to produce a non-foamed exterior layer of the desired thickness. A foamable resin is then introduced into the mold and the molding operation resumed so that a foamed layer is formed on the inside of the non-foamed layer. Such a method is disclosed in U.S. Pat. No. 3,976,821. While it is possible to produce acceptable molded goods in this manner by proper selection of resin(s) and operating conditions, the procedure is labor intensive and time-consuming. Also, it requires use of a mold with an opening to permit introduction of the foamable resin.
Another approach has been to utilize specially designed equipment, such as disclosed in U.S. Pat. No. 4,952,350, which permits both the non-foamable and foamable resin to be introduced at the beginning of the operation but maintained separately. In this way, the foamable resin can be released, e.g., from a dump box, at some point in the operation after the non-foamable resin has melted and uniformly coated the interior surface of the mold.
In other cases, such as in U.S. Pat. No. 2,989,783, the foamable resin is enclosed in a thermoplastic bag. The foaming or release of gas from the foamable resin forces the bag to expand to the shape of the mold so that the bag forms the outer surface of the article.
Still other approaches have been suggested to produce acceptable rotomolded goods having a smooth skin layer and foamed inner layer in a "one-step" process. These procedures include, for example, processes which rely on density differentials of the foamable and non-foamable resins. However, since density differences of most of the commonly used thermoplastic resins are slight, it is not possible to achieve sharp separation of the foamable and non-foamable layers using this approach.
Another method disclosed in U.S. Pat. No. 3,962,390 relies on the different heat capacities of the foamable and non-foamable resins. By using a resin having a greater heat capacity for the foamable resin, particles of the resin with the lower heat capacity begin to melt first and thereby coat the inner surface of the mold before the resin with the foam agent begins to melt.
While all of the above methods are capable of producing foamed articles with an exterior surface of reasonable quality under optimum processing conditions, with certain resins they are still prone to surface pitting, i.e., the presence of surface pores or pinholes. Surface pores, when sufficiently large and/or present in large numbers can severely detract from the appearance of the molded good and render the article unacceptable. Also, in other instances where the non-foamed skin resin is colored and the foamable interior resin particles is uncolored, surface blotches or blemishes are evident as a result of the interior foamed resin "pushing through" the skin layer. This is particularly true where a thin skin layer is desired and where, as is most usually the case, the foamable resin is in the form of pellets. Obviously, this latter problem could be overcome by coloring the foamable resin but this would increase the cost.
These and other disadvantages associated with heretofore known process are overcome by the composition and process of the present invention whereby it is possible to produce foamed rotomolded articles having an exterior skin which is substantially free of surface defects, such as pitting and color blotches, and a foamed interior.